Capacitative energy storage in EDLC uses the charging of the so-called double layer at an electrode/electrolyte interface where polarization of the electrode induces a rearrangement of ions having the opposite charge sign in the immediate vicinity of the electrode. Depending on electrode materials and electrolyte chemistry, typical double layer capacitance values range from 10 to 50 pF/cm.sup.2 on metals. In double layer capacitors, the capacitance arises by separation of electron at a metal or carbon electrode surface and ionic charges in the immediately contiguous electrolytic solution. Because the charge separation arises over only a distance of 2 to 5 .ANG., large specific capacitance values can be developed, i.e., 10-20 .mu.F/cm.sup.2 of the electrode interface.
An EDLC cell is composed of two high specific area electrodes and the overall capacitance is given by 1/C.sub.cell =1/C.sub.1 +1/C.sub.2, where C.sub.1 and C.sub.2 are the capacitances of the first and second electrode respectively. Ideally, C.sub.1 equals C.sub.2 so that the capacitance of the cell is one half that of an individual electrode and the specific capacitance, per g of the cell is one fourth of the specific capacitance of an individual electrode, e.g., C=50 F/g of electrode material having theoretical 200 F/g specific capacitance.
EDLC devices usually exhibit a set of unique properties having an effect on their performances. The most important feature is the frequency dependence of the capacitance and equivalent series resistance (ESR). Such behavior reflects the distributed nature of the double layer capacitance in relation with the porous structure of the electrodes. Consequently both the capacitance and ESR decrease with increasing frequency. At high frequency (short times), only the exterior surface or large pores of the carbon is available for charge or discharge. At lower frequencies, current penetration into the porous structure becomes progressively deeper until all the electrode surface area is accessed at very low frequency. This directly impacts the practical capacitance in relation with the charge/discharge rate.
Compared to batteries, the charging/discharging process of capacitors are virtually energy loss free. Another consequence of the non-faradaic nature of EDLC charging is that there is virtually no limitation in cycling life provided the materials used are chemically and environmentally stable. Besides the capacitance value of an electrode material, the charging/discharging kinetics of the double layer is very important as it directly influences the power capabilities of the assembled capacitor. The kinetics of porous carbon electrodes have been studied by Soffer et al, and such studies have shown that the electrode microstructure is key to fast charging/discharging processes as it directly influences the pore size distribution, the mean pore diameter and length, and the conductivity of the carbon phase.
Specific surface area and pore size distribution are important parameters to control in order to optimize the specific capacitance, per g of the electrode material. It has been shown that specific surface area up to 2000 m.sup.2 /g are linearly correlated to specific capacitance (F/g). Since high area conducting carbon materials are available with specific area values up to 2000 m.sup.2 /g, very large specific capacitance g.sup.-1 can theoretically be achieved. For example, for a specific area of 1000 m.sup.2 /g, the specific capacitance values up to 200 F/g can ideally be obtained. In practice, these high capacitance values are not realized due to physical and chemical limitations that still need to be better understood. It has also been shown that the pore size distribution of activated carbon materials has an effect on the temperature dependence of capacitance, (C). Results have shown that a desirable microstructure is one that yields high specific surface area while maintaining a low contribution of ultramicropores (d&lt;2 nm) to the total pore volume. However, measured capacitance values for such microstructures have been below the expected 10 to 20 pF/cm.sup.2, with the reported high value for specific capacitance of activated carbon fiber electrodes.sup.13 being only 6.9 .mu.F/cm.sup.2. At carbon electrodes, polarization is said to induce a semiconductor type space charge capacitance. This additional capacitance has a low value and dominates the capacitive charging process.
Marked effects of surface chemistry of porous carbons on double layer charging has been observed. For example, solvent cleansing or thermal treatments in controlled atmospheres have shown variations in capacitance values and/or in cathodic and anodic charging behaviors. Another acknowledged effect of carbon surface chemistry is the correlation of the current leakage of a capacitor with the fraction of surface acidic functional groups. Heat treatment (1000.degree. C.) of activated carbon fibers under N.sub.2 reduces the amount of acidic groups and lowers the leakage current of the capacitor in the charge state. A direct consequence of this effect is that phenol resin based carbon fibers are preferred to pitch, cellulose or PAN fibers having high contents of acidic groups.
Supercapacitors with high capacitance i.e. anywhere from 1 F to 1500 F are becoming increasingly important as energy storage devices for various applications such as consumer electronics (low back up currents 1 mA or less) needed in CMOS, RAMs, Clock ICs in consumer electronics and microcomputers; for secondary power sources or for starting small electric motors (up to 50 mA); and actuators or primary power sources for transient needs(up to 1 A). The development of large specific energy and power capacitors is said to open up a new range of potential applications, including hybrid and electrical vehicles, car engine cranking, cold start for exhaust control devices, utility load leveling, internal combustion engine starting, and many others.
Many of today's commercial supercapacitors are based on activated carbon electrodes with aqueous or organic electrolytes. Typically the construction of such electrodes is based on either activated carbon fibers or activated carbon powder. From the fibers an electrode is made by fabricating a sheet of fibers via papermaking techniques. The sheet is then plasma sprayed with aluminum to form a current collector. A tubular capacitor is then formed by spiral winding such sheets with a polymeric separator in between. When the electrodes are made from powders the fabrication typically consists of mixing the powders with binders and then coating this mixture on a metallic current collector sheet. The electrodes thus fabricated are then packaged with separators, typically a polymeric thin porous film, and in appropriate containers made of stainless steel or other material. Attempts are continually being made to increase the power density of these capacitors by optimizing the carbon nanostructure and surface chemistry as well as increasing the packing density of activated carbon powders and minimizing the amount of space occupied by the current collectors separators and other components so that more active material is packed in a given volume or for a given weight.
Accordingly, it is the object of the present invention to provide an activated carbon electrode which overcomes many of the above-stated shortcomings of fiber and powder-based activated carbon electrodes.